


Mate Him Not Me!

by Nillas NSFW Corner (Agraulis_vanillae)



Category: Undertale (Video Game)
Genre: Double Penetration, Keeper Sans, M/M, Multi, Naga Papyrus, Other, Possessiveness, Primal Papyrus, Snakes, Swapcest - Freeform, Swapfell Papyrus - Freeform, Swapfell Sans, Territoriality, Underfell Papyrus, Underfell Sans, Underswap Papyrus, Underswap Sans, i think, nagas, orgy?, swapfellcest - Freeform, w.d. gaster - Freeform
Language: English
Status: Completed
Published: 2017-07-10
Updated: 2017-07-10
Packaged: 2018-11-30 05:14:39
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,248
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11456730
Author URL: https://archiveofourown.org/users/Agraulis_vanillae/pseuds/Nillas%20NSFW%20Corner
Summary: Black has a difficult time with Slim once he hits puberty, and enlists Blue's help with his naga, Swap. Things do not go as planned.





	Mate Him Not Me!

“GET. OFF. OF ME. YOU AWFUL CREATURE!” 

 

Black shrieked as he tried to push off the overly amorous serpent that was his naga. Unperturbed by the scathing tone that his keeper used, Slim merely slid out from between his legs and the coils relaxed around him. Black could feel a headache coming on.

 

If he was absolutely pushed to it, he might admit that Slim grew into a handsome specimen of a naga. Long sleek body, deep glimmering wine-colored scales that bordered on black in just the right light, the pupils in his pale orange eyelights were wide and alert, large enough to make Sans mistake the look in his eyes for innocence until he got into mischief to remind him otherwise. He also had a gold false fang as this mischief involved having successfully distracted Black when he was very young, only to find that attempting to eat a barbecued steak larger than he was, happened to be very bad for his fangs and he got one caught. In his struggle to vault away when Black caught him, he managed to rip off a fang and had to be replaced.

 

However, that beauty was tempered by how horny the serpent was. It felt to Black like the instant Slim went through naga puberty- which he should’ve realized was going to be a issue sooner or later- he went from the relaxed albeit clingy snake that Black had gotten used to being shadowed by, to constantly attempting to curl around and mount him. Any friends that he’d been able to invite home were now chased away, Slim’s territoriality and powerful coils keeping them from being able to safely visit.

 

Frankly, he was getting sick of it. He did love the possessive asshole, as much as he’s preferred to simply  _ not,  _ it didn’t seem to be an option for some strange inexplicable reason that Black couldn’t riddle out. Yet, he’d really like to wake one day without his naga having already lockpicked his way out of the cage and attempting to curl around him in a mating ball.

 

He eventually phoned the head of the naga conservation program that had trained all of the keepers, a instructor named W.D. Gaster. Honestly, he was a huge nerd, but if there was anyone who’d have an idea what to do with problematic nagas it would be him. Hesitating only a little, his pride protesting with every dial tone that sounded, he hit call.

 

One ring, two rings…

 

“C’MON YOU OLD COOT, PICK UP ALREADY.” He complained aloud, Slim slithering over to loom over and listen. The phone picked up and Gaster’s greeting was drowned out by Black’s shouting, “NO SERIOUSLY, GO AWAY YOU HORNY MUTT.”

 

“Well that’s not very nice.” Gaster remarked, and Black groaned.

 

“NOT YOU, MY NAGA- AND I TOLD YOU IF YOU’RE GOING TO FOLLOW ME EVERYWHERE THAT YOU’RE NOT ALLOWED IN MY BED!  **_A. A. A. A. A. A_ ** !!!!” Black screamed away from the receiver.

 

“Well, it’s good to know that you two are getting along.” Gaster commented dryly. “So what brings the pleasure of your company today? I thought you weren’t going to talk to me again after barely earning a C- in class.”

 

Black winced, counting his lucky stars that Gaster couldn’t see what was happening over the phone. “MY NAGA, SLIM, HAS BEEN CONSTANTLY PESTERING ME AND I DON’T KNOW HOW TO MAKE IT STOP.”

 

He could hear a slight ‘tsk’ over the phone. “How old is your naga by this point? Juvenile? Or older?”   
  


“OLDER. OLD ENOUGH TO BE MATED, AND ACTS LIKE HE MIGHT DIE IF HE DOESN’T FIND A MATE RIGHT AWAY.” Black reported, narrowing his eyes as the snake leaned up against him. Fortunately, this time Slim seemed content to sling himself over his shoulders and nothing more, even if his weight made remaining upright a challenge. However, that was nothing that someone as maleficent as him couldn’t handle!

 

“Your solution seems fairly obvious.” Gaster commented, and the ridges of Black brows flew up in a dubious expression.

 

“IT IS?”

 

“Yes, you simply need to call up some of your old classmates and see if any of their naga are approaching heat. Then you can redirect Slim’s ‘frustration’ onto a more appropriate target.” He reasoned. “Do you need numbers? I do recall you being a little socially awkward.”

 

“TAKE THAT BACK YOU OLD COOT, I’M NOT SOCIALLY AWKWARD! I’M JUST TOO COOL FOR SCHOOL!” Black rebutted angrily and hung up on Gaster’s chuckling. Even Slim seemed more amused than impressed by that declaration, even as he crept closer to his pelvis. Black stomped his foot in frustration, “STAHHHP!!!”

 

~

 

Thus began Black’s search to get Slim a snake boyfriend- or girlfriend. His first thought was Edge, who took a no nonsense attitude towards any matter that was naga. Unlike Black, he managed to scrape together a hard earned B+ and managed to nurse one of the most sickly naga hatchlings that Black had ever seen into a fairly healthy though delicate adult.

 

However, when they’d introduced the nagas, Red had taken one look at the larger snake and hissed, hiding behind Edge. Pinching the bridge of his nose, Edge stepped aside to reveal Red who in his panic turned tail and slithered away, with Edge yelling after him “GET BACK HERE AND MATE LIKE A MAN!!!”

 

Then when he took Slim to go see Sans who was the top of the class, but an unbearable nuisance given that he punned non-stop and smelled overwhelmingly of ketchup. Yet, Slim had only taken one look at the sweetness that was Papyrus, and patted him gently before he went to go bug Black again.

 

Fed up, he even called Blue who he’d originally avoided arranging a meeting. Blue didn’t pun and smell of ketchup, but he was also one of the students that Black simply didn’t understand. Peppy and cheerful, he was the next in rank in the keepers. Yet, he always seemed to be stumbling along with making mistakes like accidentally crushing the practice egg they ‘raised’ during class, so that Black never did figure out how he could pull ahead. 

 

“YOU’VE BEEN HAVING TROUBLE WITH YOUR NAGA TOO? I THOUGHT I WAS THE ONLY ONE!” Blue declared, sighing in relief. “WHEN DO YOU THINK YOU CAN BE OVER?”

 

“M-MAYBE IN A WEEK, BUT HOLD ON!!! WHAT DO YOU MEAN YOU’VE BEEN HAVING TROUBLE?!! WHAT HAS YOURS BEEN DOING?”

 

“HE… GOT OUT WHEN I WENT INTO HEAT.” Blue admitted. Black remained silent, already picturing it. He already couldn’t get away from Slim but heat? He wouldn’t get out of Slim’s enclosure for a week. Interrupting Black’s grimace and Slim’s curious look, Blue finished saying, “...HADN’T BEEN ABLE TO CONDITION HIM TO EVEN APPROACH ANYONE ELSE SINCE.”

 

“THAT SEEMS LIKE A PROBLEM.” Black managed out.

 

“YES! SO HOPEFULLY WE CAN KNOCK OUT TWO BIRDS- ER, SNAKES? WITH ONE STONE!”

 

~

 

The nagas circled each other warily, uncertain about each other. Swap radiated a dominant body language that signified to Slim that he was far too used to his habitat to back away from the new intruder. Slim was used to his keeper taking him to meet new nagas, but not one almost twice his weight and girth though his scent told him they were close in age. He wondered what was going through the other snake’s mind, his expression were kept carefully neutral as he assessed the other snake.

 

“DO YOU THINK THEY’LL REALLY GET ALONG?” Black muttered, watching them scope each other out. His potential mate was normally better at hiding his nerves than this, the tension in his stance forcing a stiff gait once he started pacing in circles. 

 

“MWEH HEH HEH! OF COURSE, MY NAGA CAN CHARM ANYONE!.. WHEN HE FEELS LIKE IT...” Blue murmured back, chewing on his thumb bones in an offhanded nervous gesture. The honey-gold naga’s scent was all over this keeper, telling him all he needed to know about their relationship. Slim slowly gesticulated in the wordless language that nagas knew.

 

_ Your mate? _

 

The other naga seemed to relax somewhat, with a slow nod.  _ You have not been mated. I am supposed to mate you. _

 

He looked to Black, and silently Swap moved close to him. He looked to Swap again, wary of the sudden proximity, but the other snake wasn’t advancing any further. Swap gestured,  _ My mate didn’t specify who with. _

 

When Swap offered his hand, Slim hesitantly asked,  _ Are you saying what I think you’re saying? _

 

_ Just follow my lead. _

 

He placed his hand in Swap’s, only for Swap to tug him close and wrap his tail around his body, Slim nearly vaulted back in alarm only to hear Blue’s gasp from behind them. “BLACK!! LOOK!” 

 

Swap winked.  _ It’s all about the show. _

 

Slim wrapped his tail around Swap in response, thinking of Black and feeling the other naga’s hands trace around his scales, hips, and then grasping his hands to place them at his ribs. He rubbed absently at the bone, and Swap pulled him on top of him with a satisfied grunt. Encouraged, he nipped at his collarbone, and felt the coils around him tighten.

 

“THEY’RE ACTUALLY… DOING IT?” Black’s voice sounded, suddenly perplexed. “I DIDN’T THINK IT’D WORK…”

 

“I TOLD YOU MY NAGA COULD HELP!” Blue responded excitedly. Swap glanced pointedly over his shoulder, and smirked. Slim cast a sidelong glance over to Blue, seeing that the other keeper seemed engrossed in their actions. Swap licked his cheekbone, grazing the ridge of his nose over his before guiding him into a open mouthed kiss.Their coils were pulling up and twisting more heavily and Slim sighed shakily to the shocks of heat that travelled through his spine.

 

_ It’s starting to hurt, isn’t it? _

 

Knowingly, Swap pressed a palm to the swelling where his scales separated, and his digits brought his hardening members out with a long languid stroke. 

 

_ A little help? _

 

Slim could feel the bulge against him, and slipped his fingers into the opening to ‘V’ and allow Swap’s pulsing dicks to jut out and press against his bare scales. Swap’s breathe came out in a ragged huff as he rutted against him. 

 

“ABOUT TIME! FINALLY... I SHOULD UH, GET A DRINK OR SOMETHING. YES! I’M VERY THIRSTY.” Black responded, his footsteps retreating and Slim attempted to break from the coils only for Swap to hold him with a slight shake of his head.

 

_ He won’t be gone for long. Let’s give him a treat to come back to. _

 

That’s when the scent hit him. Black was aroused.

 

~

 

The flow of cold water in the bathroom sink ceased as Black turned the knob, and pressed the fuzzy washcloth to his wet face. He pressed hard against his face a few times before putting it aside and walking through the hallway back out to the enclosure.

 

He didn’t know what was up with himself, or with Blue for that matter. The other skeleton’s eyes never left them for a moment and if he hadn’t known better, he would’ve thought that Swap was twisting the way he was on purpose. The position had both snakes fully exposed, Slim already dripping pre from both members in arousal, eyes half-lidding as the honey-colored naga stroked. The mating was much slower and languid than he’d expected, their tails intertwined sensually. It made him feel light-headed as he’d held his breath subconsciously as he’d watched.

 

He pressed his back to the door that led to the enclosure, his soul beating quickly. Why couldn’t he calm down? It’s just his freakin’ naga mating! 

 

“H-HEY, WHAT ARE YOU DOING..?” He heard Blue through the door. He was still watching them, was he actually going to try to direct the mating? “NO YOU’RE SUPPOSED TO BE MA-AAAH..?!”

 

The statement trailed off oddly as if Blue was distracted, and Black’s heart rate picked up again. Oh no, he left Slim alone with Blue and Swap! Black opened the door in a rush and ran to the large enclosure with the unlocked cage door, suddenly worried. Blue wasn’t safe- he could be-

 

Sandwiched between the two snakes?!

 

Blue was flushed and squirming against the dicks that was pressed against him, Swap slowly entering him and Slim grinding from behind him. He locked eyes with Black, gaze smoldering into his eyelights and unravelling from the coil, he approached Black. He stopped just at his feet, ribcage brushing the soft grass inside the enclosure and his face right at knee height. The posture seemed to be akin to kneeling, with Slim’s face turned up towards his, all drool dripping past his fangs and hazy eyelights. Black felt him beseeching, his arms circled around him but not closed.

 

“...mm...lor’…”

 

“WHAT?” Black barely dared to breathe, very aware that if he glanced past, that the other snake was still fucking Blue with lewd squelching sounding underneath the gasps and panting. “AH~ B-BLACK!! THEY’RE- HMPH!!” His naga muffled him with a demanding kiss, and gave another thrust that nearly forced Black’s attention away if it weren’t for another breathy-

 

“m’lord.”

 

His soul felt like it was pounding out of his chest, magic rushing despite the fact that he never gave it permission to betray him this way to the pleading look that Slim was giving him as he pressed his face to his shorts. “NO FAIR…”

 

Slim paused, eyes wide.

 

“... HOW… AM I SUPPOSED TO SAY NO WHEN YOU’RE SAYING A THING LIKE THAT?”

 

Black’s soul pulsing, the light was beginning to bleed through his clothes and the two were at a standstill, caught in the moment. Then, they were swept in a long coil of Swap moving around them and dragging them into one large ball. His back was pressed to Blue’s back, he could feel both snakes all around him, smooth scales gliding underneath phalanges before his hands was taken up possessively by Slim in a pressing searing kiss that had him all but swooning. Honey and wine coils all around them, his sense of direction was almost completely lost and Slim’s face migrated up past his pelvis -shorts being tugged down and bare pelvis being nipped and licked in the process- and all the way up into his ribcage where his soul lay beating. 

 

A tongue travelled up to caress his soul, and a bright flush warmed his face as his hips twitched of their own accord, brushing against something hot and foreign. The angle should’ve been wrong for Slim to be entering him, but impossibly there was something pushing up against his exposed entrance, slickness sliding up his legs and slowly spreading him open. He allowed his head to lull, seeing a shift in the gaze of the honey colored naga to his eyes.

 

An angry hiss sound from his ribcage, his soul being seized perilously between a fanged mouth and Blue audibly gasped as his naga separated from their kiss to stare coolly down at him. No, not him, his gaze was past Black’s face and his face burning he knew that he must’ve been making eye contact with Slim. His soul’s glow was soon extracted from his ribcage, the red-purple slowly oozing in rivulets down his face. His eyelights went from slits to completely black leering back at the other naga. That was when Black felt something else at his entrance, throbbing and insistent. Alarmed, he cries out, “W-WAIT!!!”

 

“B-BLACK THEY’RE NOT GOING TO WAIT LIKE THIS, THEY’RE IN A MATING BALL! AND THEY’RE TRYING TO-OHHH GEEZ…” Blue tried to explain in a hurry but was cut off in a low pant. Black threw his head back too as he was abruptly penetrated by Slim bucking into him in a hurry. “THEY’RE COMPETING TO MATE US BOTH!”

 

“I CAN FUCKING TELL!!” The way the coils around him had gone from languid to writhing with intense fury, not to mention that he could barely fit sheathed over Slim but Swap’s second cock was also pushing into him insistently was a pretty damn big giveaway. He cried out at the pressure increasing, “AH, TOO MUCH TOO MUCH!”

 

Swap pressed in all the way inside despite the burning pain that built up in Black’s eyes and dripped down his face, and Slim was hissing once more in territorial irritation, the sound squeezing around his soul and making white starbursts behind his vision. The pain of being stretched out so much was only adding to the overwhelming sensation of both snakes driving inside of him, the shapes of their cocks long and slender but with the heads both snagging beyond each other with a popping movement, and he couldn’t avoid the “HAH~!” that had fell from his throat. Slim was deepest inside him with a particularly brutal thrust, forcing whimpers from Black and both keepers to jostle against each other before Swap pushed back shallowly inside Black. Blue was grabbing behind him desperately for something to hold and grasped Black’s collarbone with his left hand with a long keening wail that left no room for questions as to the meaning of it.

 

Both snakes then thrust again and a new dripping wet was seeping against Black with hot frictionless movement. Blue was still holding his collarbone, anchoring them both against each other against the harsh mating. The tips of his phalanges dug underneath the bone, sending sharp pains through his head. His vision tunneled to the soul that Slim mouthed softly, pressure mounting through his body as an unfamiliar keening sang through the air with a soft mutter.

 

It was only a few moments later that he realized that was his voice, the mutter was a breathless Blue, “TOO MUCH TOO MUCH TOO MUCH-”

 

The back against his was pressing against the top of his spine, curling. If Slim hadn’t grabbed his hands and held them in his own death grip, maybe Black would’ve grabbed for Blue’s hips or ribs behind him. He was pushed forward into Slim from the force of Swap’s returning push, his face close to Slim who stared into his. His tongue roved across the soul to peek out from between his fangs and with another soft muffled sound he spoke,

 

“m’lord… mine. mine. mine. mine.”

 

Somewhere in-between the rocking of both members inside him and that hungry worshipful gaze never leaving his eyes, the pressure finally broke and he was hit with an intense heady orgasm that had swelled and ebbed like a tidal wave and seemed to last forever, his pulsing cock shooting cum full into his own face before he passed out. 

 

~

 

He woke up sore as hell and completely surrounded by a softly snoring Slim in what appeared to be the guest bedroom that Blue had prepared for him the day before. In a sudden rush of awareness that his soul was in the serpent’s mouth, he felt his ribcage and summoned his soul- intact. Thanking his lucky stars, he relaxed back into the coils for a few moments. 

 

Eventually, he finally eased out of bed again, barely restraining a wince at the sharp shooting pain that laced through his spine. Blue was already awake and downstairs, sipping away at coffee which was more white than brown with as much cream as there was in it. Black took one look at the cup and decided that he’d rather drink his coffee black. He sat down with a wince after retrieving his own cup, silence reigning.

 

“SORE…” Blue murmured into his cup. Black simply nodded.

 

“NEVER AGAIN.”

**Author's Note:**

> This took me forever to publish, I had originally wanted to get this out about a month and a half ago but life got in the way. Tougher to write and work than I thought. Anyways, I hope this was okay! I'd been meaning to write a full naga oneshot for awhile. If I missed anything, it's probably 'cuz I've been writing this in tiny segments the last month or so and have been half asleep all week.  
> Feel like following me? Here's my tumblr.  
> https://agraulisvanillae.tumblr.com/


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